It is well known that contact lenses can be used for cosmetics and the correction of visual acuity. The ideal contact lens is one which is not only comfortable to wear for extended periods of time, but also easily and reproducibly manufactured at minimum cost in time and labor.
Contact lenses can be manufactured economically in large numbers by the so-called mold or full-mold process. Known contact lens-molding processes are described in, for example, PCT patent application no. WO/87/04390 or in EP-A 0 367 513. In a typical molding process, a predetermined amount of a polymerizable or crosslinkable material is placed in the female mold half and the mold is closed by placing the male mold half proximately to the female mold half to create a cavity having a desired geometry for a contact lens. Normally, a surplus of polymerizable or crosslinkable material is used so that when the male and female halves of the mold are closed, the excess amount of the material is expelled out into an overflow area adjacent to the mold cavity. The polymerizable or crosslinkable material remaining within the mold is polymerized or cross-linked with the delivery of radiation thereto through UV light, heat action, or another non-thermal methods. Since the geometry of the ophthalmic lens is specifically defined by the cavity between the male and female mold halves and since the geometry of the edge of the ophthalmic lens is defined by the contour of the two mold halves in the area where they make contact, a contact lens is manufactured into a final form between typically male and female mold halves, with no additional finishing work on the surface of the lens or the edges of the lens. Such full-mold process can reduce cost in the production of contact lenses. However, In a typical molding process, a contact lens, which is removed from the mold after curing, needs to undergo the other manufacturing processes such as hydration/extraction and sterilization. Therefore, there is still room for further reducing manufacturing cost of contact lenses.
U.S. Pat. Nos. 5,508,317, 5,583,463, 5,789,464, and 5,849,810 describe an improved manufacturing process for economically producing contact lenses in large numbers. By using a water-soluble composition of a prepolymer which is a water-soluble photo-crosslinkable polyvinyl alcohol, a finished lens of optical quality can be produced in a mold within a few seconds without the necessity for subsequent extraction or finishing steps to the contact lens. With such manufacturing process, contact lenses can be manufactured at considerably low cost and thus it is possible to produce disposable contact lenses that are discarded by the user after a single use.
Contact lenses manufactured by the process of the U.S. Pat. No. 5,583,163 have advantageous properties such as a good compatibility with the human cornea resulting in a high wearing comfort and the absence of irritation and allergenic effects. However, despite the good mechanical stability of the underlying polyvinyl alcohol material problems may sometimes show up in production. In particular, during mold opening and removing the contact lenses from the mold, cracks, flaws or tears may occur in the lenses or in the worst case the contact lenses even break totally. Contact lenses having such defects have to be discarded and lower the overall production yield.
In addition, contact lenses made from a water-soluble photo-crosslinkable polyvinyl alcohol prepolymer do not always posses all of most desirable physical properties, for example, such as elasticity and durability, for the intended uses.
Accordingly, there is still a need for a new prepolymer for economically producing durable, highly elastic soft contact lenses with desired physical properties.